Heretofore methods of applying fixed process parameters to the processing of polymeric rubber compounds during vulcanization have resulted in both reduced productivity due to overly conservative cure times and poor product uniformity due to the inability of the fixed process parameters to accommodate the inherent variability in the process.
The relationship of dielectric properties and the state and rate of the cure of polymers well known. Related publications in this field include:
U.S. PATENT DOCUMENTS4,344,142August 1982Diehr,II et al.4,373,092February 1983Zsolnay4,399,100August 1983Zsolnay, et al.4,423,371December 1983Senturia, et al.4,496,697January 1985Zsolnay, et al.4,510,103April 1985Yamaguchi, et al.4,551,807November 1985Hinrichs, et al.4,723,908February 1988Kranbuehl4,777,431October 1988Day, et al.4,773,021September 1988Harris, et al.4,868,769September 1989Persson, et al.5,032,525July 1991Lee, et al.5,219,498June 1993Keller, et al.5,317,252May 1994Kranbuehl5,486,319January 1996Stone, et al.5,528,155June 1996King, et al.5,872,447February 1999Hager, III
Other Publications                Changes in the Electrical Properties of Natural Rubber/Carbon Black Compounds during Vulcanization, 1957, H. Desanges, French Rubber Institute        A novel method of measuring cure—dielectric vulcametry, 1986, Sture Persson, The Plastics and Rubber Institute, England        A comparative study of step curing and continuous curing methods, 1994, D. Khastgir, Indian Institute of Technology        AC Impedance Spectroscopy of Carbon Black-Rubber composites, 1999, K. Rajeshwar, University of Texas at Arlington        
The prior art has clearly established the relationship between the dielectric (herein also referred to as “impedance”) properties of polymeric resins which exhibit rheometric and chemical behavior such as melt, volatile release, gelation, and crosslinking that can be recognized by dielectric means for those skilled both in the art and those resins' physical properties. However, unlike polymeric resins, polymeric rubber compounds do not melt or exhibit gelation during cure or vulcanization and are therefore much more difficult to characterize, monitor and control by dielectric means. Moreover, none of the prior art associated with polymeric rubber curing (also referred to as “vulcanization”) addresses the practical aspects of taking measurements directly in the production process, especially in the highly abrasive and high pressure environment of injection molding. Additionally the prior art does not show how to use the electrical data obtained to achieve closed-loop control of the curing or vulcanization process over a wide range of molding methods and conditions.
The prior art also does not show how to compensate the vulcanization process for variations in compound from batch to batch and within batches, and for differences in vulcanizate thickness. Additionally, the prior art does not compensate for additional variables, which are introduced into the process by the nature of the vulcanization equipment, tooling, and thermal history of the compound.
Moreover, the prior art uses dielectric or impedance measuring apparatus, which employ opposing and parallel electrodes of precise area and separation distance, and in which, the electrodes are in direct contact with the rubber compound. Although such electrodes and apparatus provide a means for measuring impedance properties during cure, they are entirely impractical for use in a production environment. For example, many rubber components are produced using injection molding technology which subjects the sensors to pressures up to 30,000 psi and temperatures up to 425° F. Moreover, due to the flow inside the mold during injection, in addition to the carbon and silica fillers present in many rubber compounds, the sensor must survive in a highly abrasive environment. Finally, the sensor must also be able to survive mold cleaning via typical cleaning methods such as CO2 and plastic bead blast.
Accordingly, it is desirable to have an apparatus and method for alleviating the above described drawbacks to using impedance data measurements for monitoring and controlling the vulcanization process. In this case, the impedance sensor provided at the vulcanization equipment is both extremely rugged and more easily used in that the electrodes need not be of precise area, need not be of precise separation distance from one another, need not be in direct contact with the material being vulcanized. In addition, a method is established for correlating the desired properties of the rubber product with the impedance measurements.
Definitions and Terms
Numerous technical terms and abbreviations are used in the description below. Accordingly, many of these terms and abbreviations are described in this section for convenience. Thus, if a term is unfamiliar to the reader, it is suggested that this section be consulted to obtain a description of the unknown term.                Rubber Polymeric Compounds: Typical base rubber polymeric compounds which may be employed include styrene-butadiene, polybutadiene, polyisoprene, ethylene-propylene, butyl, halobutyl, nitrile, polyacrylic, neoprene, hypalon, silicone, fluorcarbon elastomers, polyurethane elastomers, and mixtures thereof.        ODR: Oscillating Disk Rheometer—Device that measures the rheological characteristics (elastic torque, viscous torque, etc.) of a polymer during vulcanization, using an oscillating disk to apply stress to the curing polymer.        MDR: Moving Die Rheometer—Device that measures the rheological characteristics (elastic torque, viscous torque, etc.) of a polymer during vulcanization, using a moving die to apply stress to the curing polymer.        Rheometric instrument: Device that measures the rheological characteristics (elastic torque, viscous torque, etc.) of a polymer during vulcanization.        T90 Time: The time, as measured in an ODR or MDR at which a given rubber compound at a given curing temperature, reaches 90% of its ultimate elastic torque value.        Designed Experiment: A single set of actual related experiments drawn up from one of the types of designs to be found in the body of methods for design of experiments.        Exponential Dampening: The damping coefficient (α) as defined by a best exponential fit to a set of raw data, where the fit curve (y) is described by the equation y=Aeαt, where t is time.        Exponential Amplitude Coefficient: The amplitude coefficient (A) as defined by a best exponential fit to a set of raw data, where the fit curve (y) is described by the equationy=Ae−αt, where t is time.         Topological Features of Impedance Related Data: Recognizable and distinct features within a cure curve, such as a peak (maxima), valley (minima) or flat (no slope).        Low CTE Metallic Material: Material with low coefficient of thermal expansion.        Tool Steel: A steel suitable for use in making injection and compression molds such as AISI Type A2 Tool Steel.        Witness cavity: A small cavity for in-mold vulcanization measurement, whereby the dielectric sensor does not directly sense any of the parts that are being produced. Instead, the sensor monitors the cure in the “witness” location.                    R-square (R2): R-square (also known as the coefficient of determination) is a statistical measure of the reduction in the total variation of the dependent variable due to the independent variables. An R-square close to 1.0 indicates that the model (as used herein, the algorithm) accounts for almost all of the variability in the respective variables.                        Confidence interval: A range of values within which a particular number of interest is calculated to fall, at some specific level of probability such as 95%.        